Traditional speech network circuits for telephone sets have been constructed with a hybrid transformer, designed to provide some degree of isolation between signals that are simultaneously transmitted and received over a two-wire telephone line. The elimination of the hybrid transformers by using an electronic network can result in a considerable size and weight reduction, while providing the opportunity for improved telephone system performance.
Various such electronic networks have been proposed in the past, some utilizing the gyrator principle while others are based on a bridge configuration, to provide the necessary isolation and anti-sidetone network for the telephone set. One such network which is based on this latter concept is disclosed in Canadian Pat. No. 843,513 entitled "Non-Reactive Anti-Sidetone Network For A Telephone Set", invented by Roger E. Holtz, issued June 2, 1970. One requirement of any telephone set is that it must be able to function in parallel with the conventional hybrid-transformer telephone in long-loop (rural) applications. However, because of its heavy current drain, the latter type reduces the available voltage on the line to the point where it has been found difficult to operate the electronic set. Compounding this problem is the requirement that the electronic set must draw sufficient current when in an off-hook condition to actuate the central office equipment which places dial tone on the telephone line.
In a typical short loop application, the telephone draws about 100mA with a d-c voltage drop of about 10 volts thereacross. To limit power in an electronic network, a portion of this voltage drop may be provided by an external resistor (of about 40.OMEGA.) connected in series with the telephone line which in conjunction with a shunt connected capacitor also provides some surge protection for the electronic network.
However, in long loop applications the telephone is limited to a maximum d-c voltage drop of 5 volts in order to draw sufficient current (about 30mA) from the central office to actuate the line relay which senses when the telephone set has gone off-hook. This results in a 1.2 volt drop across the external 40.OMEGA. resistor and with a further 1 volt drop across a diode bridge-connected polarity guard, the voltage available to the balance of the electronic network is limited to a d-c voltage of about 2.8 volts. This nominal d-c voltage must be capable of swinging down to about 0.8 volts for peak negative excursions of the transmitted or received audio signals.
In parallel operation with a conventional hybrid transformer telephone on long loop applications, an electronic set would drop the line voltage below the 5 volt limit if it were permitted to take half of the available current. Therefore, the electronic network must function with the lowest possible current thereby leaving as much current as possible for the hybrid set.
An additional problem is encountered during tone signalling. In order to prevent clipping of the signals during this interval, it is necessary to increase the d-c line voltage across the set. This is possible since the holding current of the line relay in the central office is less than its pull-in current. One arrangement which can be used to achieve this increased operating voltage for the tone generator during signalling is to disconnect the microphone from its transmit amplifier, reduce the gain of the receiver amplifier and remove a d-c clamp across the telephone line (used to control the line current from the central office). In order to meet the operating requirements for the overall telephone set, this change in d-c operating voltage must take place in a period less than 10 msec. However, sufficient decoupling of the lowest audio frequencies (300 Hz) for the transmit amplifier and the d-c clamp, results in a single stage R-C time constant for the bias network which is typically greater than 1 second.